1. Field of the Invention
The present invention relates to a display device and more particularly to a liquid crystal display device in which a drive circuit is formed around a display region, and an active region of a thin film transistor is different between the display region and a peripheral circuit part.
2. Background Art
In liquid crystal display devices, a color filter substrate on which color filters and the like are formed at positions corresponding to pixel electrodes is arranged so as to face a TFT substrate on which the pixel electrodes, thin film transistors (TFTs), and the like are formed in a matrix, and liquid crystal is interposed between the TFT substrate and the color filter substrate. An image is formed by controlling optical transmittance of liquid crystal molecules for each pixel.
Data lines extending in the vertical direction and arranged in the horizontal direction, and scanning lines extending in the horizontal direction and arranged in the vertical direction are formed on the TFT substrate. A pixel is formed in each region surrounded by the data lines and the scanning lines. The pixel is mainly formed of a pixel electrode and a thin film transistor (TFT) which is a switching element. Numeral pixels formed in a matrix in this manner form a display region.
Scanning line drive circuits driving the scanning lines and data line drive circuits driving the data lines are disposed outside the display region. Conventionally, as the scanning line drive circuit and the data line drive circuit, IC drivers have been mounted externally. Some IC drivers are connected to the TFT substrate by a tape carrier method or the like; others are directly mounted on the TFT substrate as chips.
On the other hand, a technique for forming a drive circuit with a TFT around a display region has been developed because of a demand for a reduction in size of the entire display device while ensuring the display region. In such a display device, a-Si is used for a channel part in a TFT formed in the display region, and poly-Si is used for a channel part in a TFT formed in a drive circuit part. That is, a-Si having small leak current is used in the display region, and poly-Si having high electron mobility is used in the drive circuit part.
In general, the bottom gate structure is adopted in the TFT using a-Si, and the top gate structure is adopted in the TFT using poly-Si. Accordingly, TFTs having different structure must be formed on one substrate, which complicates the manufacturing process. Patent Document 1 discloses a configuration in which the TFT using poly-Si also adopts the bottom gate structure for preventing the process from becoming complicated. In this configuration, a poly-Si layer which serves as a channel is first formed on a gate insulating film formed on a gate electrode, and an a-Si layer is formed on the poly-Si layer. A contact layer formed of an n+layer is formed on the a-Si layer, and source/drain electrodes (SD electrodes) are formed on the contact layer. The TFT using poly-Si for the channel is configured as described above, so that the number of steps which can be shared in common between the TFT using a-Si for the channel and the TFT using poly-Si for the channel is increased, thereby simplifying the manufacturing process.    [Patent Document 1] JP-A-5-55570